In this specification, a turbocharger is an assembly comprising a turbine driven in rotation by an exhaust gas flow, the turbine driving a compressor which urges an intake gas to flow through a compressor flowpath. Commonly the compressor and turbine are fixed on a shaft which rotates in bearings lubricated by a supply of oil, with the compressor supplying air to an internal combustion engine while the turbine is driven by the exhaust gas flow from the engine. The tendency for lubricating oil to leak from the shaft bearings into the intake gas flowing through the compressor flowpath is known as oil carry-over.
Since the exhaust gas flow rate varies with engine speed and load, it is common to provide means whereby an engine control unit may actively control the flow of exhaust gas through the turbine flowpath so as to optimise efficiency under different operating conditions.
One way to achieve this is to vary the geometry of the turbine flowpath, commonly by means of adjustable guide vanes arranged upstream of the turbine. For example, WO2016048678A1 discloses a turbocharger in which the guide vanes are controlled via a geared actuating mechanism by an actuator disposed outside the turbocharger bearing housing to alter the inflow angle of the exhaust gas at the turbine wheel inlet. The angle of attack of the guide vanes is controlled to maximise efficiency as the flow rate through the turbocharger varies.
The exhaust gas flow may also be controlled by means of a valve arranged to divert a variable proportion of the gas in the turbine flowpath to bypass the turbine. The diverted portion of the exhaust gas may be exhausted to ambient, in which case the valve is commonly termed a wastegate.
Similar flow control arrangements may be employed in a staged turbocharger assembly, in which two or more turbochargers are arranged in series flow relation to provide a higher compression ratio. For example, U.S. Pat. No. 9,003,794 (B2) discloses staged high and low pressure turbochargers having a valve for selectively bypassing the high and low pressure turbines.